Post-glacial Crustal Movements And Climatic Changes

An interesting practical application of some of the preceding
generalizations is found in an attempt by C. E. P. Brooks[95] to
interpret post-glacial climatic changes almost entirely in terms of
crustal movement. We believe that he carries the matter much too far,
but his discussion is worthy of rather full recapitulation, not only
for its theoretical value but because it gives a good summary of
post-glacial changes. His climatic table for northwest Europe as
reprinted from the annual report of the Smithsonian Institution for
1917, p. 366, is as follows:

Brooks bases his chronology largely on De Geer's measurements of the
annual layers of clay in lake bottoms but makes much use of other
evidence. According to Brooks the last glacial epoch lasted roughly from
30,000 to 18,000 B. C., but this includes a slight amelioration of
climate followed by a readvance of the ice, known as the Buhl stage.
During the time of maximum glaciation the British Isles stood twenty or
thirty feet higher than now and Scandinavia was "considerably" more
elevated. The author believes that this caused a fall of 1 deg.C. in the
temperature of the British Isles and of 2 deg.C. in Scandinavia. By an
ingenious though not wholly convincing method of calculation he
concludes that this lowering of temperature, aided by an increase in the
area of the lands, sufficed to start an ice sheet in Scandinavia. The
relatively small area of ice cooled the air and gave rise to an area of
high barometric pressure. This in turn is supposed to have caused
further expansion of the ice and to have led to full-fledged glaciation.

About 18,000 B. C. the retreat of the ice began in good earnest. Even
though no evidence has yet been found, Brooks believes there must have
been a change in the distribution of land and sea to account for the
diminution of the ice. The ensuing millenniums formed the Magdalenian
period in human history, the last stage of the Paleolithic, when man
lived in caves and reindeer were abundant in central Europe.[96] At
first the ice retreated very slowly and there were periods when for
scores of years the ice edge remained stationary or even readvanced.
About 10,000 B. C. the edge of the ice lay along the southern coast of
Sweden. During the next 2000 years it withdrew more rapidly to about
59 deg.N. Then came the Fennoscandian pause, or Gschnitz stage, when for
about 200 years the ice edge remained in one position, forming a great
moraine. Brooks suggests that this pause about 8000 B. C. was due to the
closing of the connection between the Atlantic Ocean and the Baltic Sea
and the synchronous opening of a connection between the Baltic and the
White Seas, whereby cold Arctic waters replaced the warmer Atlantic
waters. He notes, however, that about 7500 B. C. the obliquity of the
ecliptic was probably nearly 1 deg. greater than at present. This he
calculates to have caused the climate of Germany and Sweden to be 1 deg.F.
colder than at present in winter and 1 deg.F. warmer in summer.

The next climatic stage was marked by a rise of temperature till about
6000 B. C. During this period the ice at first retreated, presumably
because the climate was ameliorating, although no cause of such
amelioration is assigned. At length the ice lay far enough north to
allow a connection between the Baltic and the Atlantic by way of Lakes
Wener and Wetter in southern Sweden. This is supposed to have warmed the
Baltic Sea and to have caused the climate to become distinctly milder.
Next the land rose once more so that the Baltic was separated from the
Atlantic and was converted into the Ancylus lake of fresh water. The
southwest Baltic region then stood 400 feet higher than now. The result
was the Daun stage, about 5000 B. C., when the ice halted or perhaps
readvanced a little, its front being then near Ragunda in about latitude
63 deg.. Why such an elevation did not cause renewed glaciation instead of
merely the slight Daun pause, Brooks does not explain, although his
calculations as to the effect of a slight elevation of the land during
the main period of glaciation from 30,000 to 18,000 B. C. would seem to
demand a marked readvance.

After 5000 B. C. there ensued a period when the climate, although still
distinctly continental, was relatively mild. The winters, to be sure,
were still cold but the summers were increasingly warm. In Sweden, for
example, the types of vegetation indicate that the summer temperature
was 7 deg.F. higher than now. Storms, Brooks assumes, were comparatively
rare except on the outer fringe of Great Britain. There they were
sufficiently abundant so that in the Northwest they gave rise to the
first Peat-Bog period, during which swamps replaced forests of birch and
pine. Southern and eastern England, however, probably had a dry
continental climate. Even in northwest Norway storms were rare as is
indicated by remains of forests on islands now barren because of the
strong winds and fierce storms. Farther east most parts of central and
northern Europe were relatively dry. This was the early Neolithic period
when man advanced from the use of unpolished to polished stone
implements.

Not far from 4000 B. C. the period of continental climate was replaced
by a comparatively moist maritime climate. Brooks believes that this was
because submergence opened the mouth of the Baltic and caused the fresh
Ancylus lake to give place to the so-called Litorina sea. The
temperature in Sweden averaged about 3 deg.F. higher than at present and in
southwestern Norway 2 deg.. More important than this was the small annual
range of temperature due to the fact that the summers were cool while
the winters were mild. Because of the presence of a large expanse of
water in the Baltic region, storms, as our author states, then crossed
Great Britain and followed the Baltic depression, carrying the moisture
far inland. In spite of the additional moisture thus available the snow
line in southern Norway was higher than now.

At this point Brooks turns to other parts of the world. He states that
not far from 4000 B. C., a submergence of the lands, rarely amounting to
more than twenty-five feet, took place not only in the Baltic region but
in Ireland, Iceland, Spitzbergen, and other parts of the Arctic Ocean,
as well as in the White Sea, Greenland, and the eastern part of North
America. Evidences of a mild climate are found in all those places.
Similar evidence of a mild warm climate is found in East Africa, East
Australia, Tierra del Fuego, and Antarctica. The dates are not
established with certainty but they at least fall in the period
immediately preceding the present epoch. In explanation of these
conditions Brooks assumes a universal change of sea level. He suggests
with some hesitation that this may have been due to one of Pettersson's
periods of maximum "tide-generating force." According to Pettersson the
varying positions of the moon, earth, and sun cause the tides to vary in
cycles of about 9, 90, and 1800 years, though the length of the periods
is not constant. When tides are high there is great movement of ocean
waters and hence a great mixture of the water at different latitudes.
This is supposed to cause an amelioration of climate. The periods of
maximum and minimum tide-generating force are as follows:

Brooks thinks that the big trees in California and the Norse sagas and
Germanic myths indicate a rough agreement of climatic phenomena with
Pettersson's last three dates, while the mild climate of 4000 B. C. may
really belong to 3500 B. C. He gives no evidence confirming Pettersson's
view at the other three dates.

To return to Brooks' sketch of the relation of climatic pulsations to
the altitude of the lands, by 3000 B. C., that is, toward the close of
the Neolithic period, further elevation is supposed to have taken place
over the central latitudes of western Europe. Southern Britain, which
had remained constantly above its present level ever since 30,000 B. C.,
was perhaps ninety feet higher than now. Ireland was somewhat enlarged
by elevation, the Straits of Dover were almost closed, and parts of the
present North Sea were land. To these conditions Brooks ascribes the
prevalence of a dry continental climate. The storms shifted northward
once more, the winds were mild, as seems to be proved by remains of
trees in exposed places; and forests replaced fields of peat and heath
in Britain and Germany. The summers were perhaps warmer than now but the
winters were severe. The relatively dry climate prevailed as far west as
Ireland. For example, in Drumkelin Bog in Donegal County a corded oak
road and a two-story log cabin appear to belong to this time. Fourteen
feet of bog lie below the floor and twenty-six above. This period,
perhaps 3000-2000 B. C., was the legendary heroic age of Ireland when
"the vigour of the Irish reached a level not since attained." This, as
Brooks points out, may have been a result of the relatively dry climate,
for today the extreme moisture of Ireland seems to be a distinct
handicap. In Scandinavia, civilization, or at least the stage of
relative progress, was also high at this time.

By 1600 B. C. the land had assumed nearly its present level in the
British Isles and the southern Baltic region, while northern Scandinavia
still stood lower than now. The climate of Britain and Germany was so
humid that there was an extensive formation of peat even on high ground
not before covered. This moist stage seems to have lasted almost to the
time of Christ, and may have been the reason why the Romans described
Britain as peculiarly wet and damp. At this point Brooks again departs
from northwest Europe to a wider field:

It is possible that we have to attribute this damp period in
Northwest Europe to some more general cause, for Ellsworth
Huntington's curves of tree-growth in California and climate in
Western Asia both show moister conditions from about 1000 B. C. to
A. D. 200, and the same author believes that the Mediterranean lands
had a heavier rainfall about 500 B. C. to A. D. 200. It seems that
the phase was marked by a general increase of the storminess of the
temperate regions of the northern hemisphere at least, with a
maximum between Ireland and North Germany, indicating probably that
the Baltic again became the favourite track of depressions from the
Atlantic.

Brooks ends his paper with a brief resume of glacial changes in North
America, but as the means of dating events are unreliable the degree of
synchronism with Europe is not clear. He sums up his conclusions as
follows:

On the whole it appears that though there is a general similarity in
the climatic history of the two sides of the North Atlantic, the
changes are not really contemporaneous, and such relationship as
appears is due mainly to the natural similarity in the geographical
history of two regions both recovering from an Ice Age, and only
very partially to world-wide pulsations of climate. Additional
evidence on this head will be available when Baron de Geer publishes
the results of his recent investigations of the seasonal glacial
clays of North America, especially if, as he hopes, he is able to
correlate the banding of these clays with the growth-rings of the
big trees.

When we turn to the northwest of North America, this is brought out
very markedly. For in Yukon and Alaska the Ice Age was a very mild
affair compared with its severity in eastern America and
Scandinavia. As the land had not a heavy ice-load to recover from,
there were no complicated geographical changes. Also, there were no
fluctuations of climate, but simply a gradual passage to present
conditions. The latter circumstance especially seems to show that
the emphasis laid on geographical rather than astronomical factors
of great climatic changes is not misplaced.

Brooks' painstaking discussion of post-glacial climatic changes is of
great value because of the large body of material which he has so
carefully wrought together. His strong belief in the importance of
changes in the level of the lands deserves serious consideration. It is
difficult, however, to accept his final conclusion that such changes are
the main factors in recent climatic changes. It is almost impossible,
for example, to believe that movements of the land could produce almost
the same series of climatic changes in Europe, Central Asia, the western
and eastern parts of North America, and the southern hemisphere. Yet
such changes appear to have occurred during and since the glacial
period. Again there is no evidence whatever that movements of the land
have anything to do with the historic cycles of climate or with the
cycles of weather in our own day, which seem to be the same as glacial
cycles on a small scale. Also, as Dr. Simpson points out in discussing
Brooks' paper, there appears "no solution along these lines of the
problem connected with rich vegetation in both polar circles and the
ice-age which produced the ice-sheet at sea-level in Northern India."
Nevertheless, we may well believe that Brooks is right in holding that
changes in the relative level and relative area of land and sea have had
important local effects. While they are only one of the factors involved
in climatic changes, they are certainly one that must constantly be kept
in mind.